A Systems Approach to Mitigate Oriental Fruit Fly Risk in ‘Sharwil’ Avocados Exported from Hawaii

A multi-component systems approach is proposed to reduce the risk of oriental fruit fly infestation in ‘Sharwil’ avocados exported from Hawaii into the United States to an acceptable level. This systems approach is based on poor host status, limited distribution and low prevalence. Recent surveys suggest oriental fruit fly populations are naturally low in Hawaii’s small avocado orchards. Maintaining low oriental fruit fly numbers in orchards during the shipping period should reduce the rate of infestation to negligible levels. Population levels will be monitored using protein bait traps and if needed, suppression will be achieved using selective reducedrisk insecticide baits. ‘Sharwil’ avocados will be shipped only to northern tier states and will only be shipped during the winter months of November to March, when temperatures in the distribution area are inhospitable to tropical fruit pests. The cumulative effect of multiple safeguards should provide quarantine security against oriental fruit fly and other quarantine pests in exported avocados. INTRODUCTION Avocados, Persea americana Miller, grown in Hawaii cannot be exported to the United States mainland without quarantine treatment for Bactrocera cucurbitae (Coquillett) (melon fly), Bactrocera dorsalis (Hendel) (oriental fruit fly), and Ceratitis capitata (Wiedemann) (Mediterranean fruit fly). The most widely grown cultivar of avocado is ‘Sharwil’, a Mexican-Guatemala hybrid that has been grown in Hawaii since 1955. Avocado growers would like to export ‘Sharwil’ fruit to United States mainland markets. Both Armstrong et al. (1983) and Armstrong (1991) reported that ‘Sharwil’ avocados in Hawaii, attached to the tree, are not naturally infested by the melon fly, oriental fruit fly or Mediterranean fruit fly at harvest maturity. They suggested that fruit could be exported safely as a non-host using a systems approach where fruit are harvested with stems attached, brought to the packinghouse within 12 h, sorted to remove damaged fruit and packed in fruit fly-proof cartons. Avocados are typically harvested at the hard, mature green stage and ripened at room temperature until they soften before consumption. ‘Sharwil’ avocado, like other avocado cultivars, becomes an increasingly favorable host for fruit flies as it ripens and softens after harvest (Armstrong, 1991; Oi and Mau, 1989). A systems approach for export of Hawaii ‘Sharwil’ avocados to the U.S. mainland, based on non-host host status, was approved by U. S. Department of Agriculture (USDA) and Animal and Plant Health Inspection Service (APHIS) but the rule was rescinded in 1992, when live oriental fruit fly larvae were found in mature green fruit attached to the tree. Subsequently, Liquido et al. (1995) studied natural infestation rates in the field and found a low level of infestation by oriental fruit fly. Therefore, ‘Sharwil’ avocado is naturally a poor host, rather than a non-host for this pest, as previously thought. No infestation by Mediterranean fruit fly or melon fly was detected. Subjective firmness measures taken in the field at harvest indicated that 'firm ripe' and 'fully ripe' fruit can a [email protected] Proc. IS on Postharvest Pacifica 2009 Eds.: E.W. Hewett et al. Acta Hort. 880, ISHS 2010 440 occur on the tree late in the season (2.2% ripe fruit) and are much more likely to be infested by oriental fruit fly than hard, mature green fruit. Recent studies of Sharwil avocado infestation by oriental fruit fly and population densities in commercial orchards suggest a modified systems approach may be feasible. Follett (2009) exposed harvest mature ‘Sharwil’ fruit in cages to high densities of oriental fruit flies and Mediterranean fruit flies and only oriental fruit fly was capable of successfully infesting fruit within 24 h of harvest. This study quantified the relationship between avocado firmness and fruit fly attack by taking fruit penetrometer readings after exposure to fruit flies. While rate of infestation was negatively correlated with fruit firmness (number of pupae = 14.13 – 0.18 (firmness [N]) (F1,95 = 112.8, P<0.001) even some hard fruit (60-80 N) were infested (Fig. 1, Follett, 2009). Klungness et al. (2009) surveyed fallen ‘Sharwil’ avocado fruit and found a low level of infestation by oriental fruit fly and a 15-month survey using protein bait traps demonstrated that adult female oriental fruit flies were present at only low levels in commercial orchards (Fig. 2). Therefore, data from Liquido et al. (1995), Klungness et al. (2009) and Follett (2009) suggest that only oriental fruit fly is a quarantine pest of concern in commercially harvested fruit in the area where ‘Sharwil’ avocados are grown and that oriental fruit fly can infest commercial grade fruit, albeit rarely. A modified systems approach was developed for export ‘Sharwil’ avocados based on poor host status, low prevalence of oriental fruit fly and limited sales distribution. The low prevalence components and limited distribution are additions to the previous protocol to further reduce the risk of oriental fruit fly introduction to the U.S. mainland. A PROPOSED SYSTEMS APPROACH There are a number of components (Table 1) of the proposed systems approach and each component is discussed below. Underpinning the approach is the understanding that ‘Sharwil’ avocado fruit are rarely infested by oriental fruit fly and that the frequency of infestation and the risk of introduction can be reduced to an acceptable level. ‘Sharwil’ Cultivar The identity of avocados must be ensured as cultivars may differ in their susceptibility to fruit flies (Hennessey et al., 1995). ‘Sharwil’ is a distinct cultivar and existing orchards and trees must be certified as ‘Sharwil’ based on horticultural characteristics. ‘Sharwil’ trees have a low, spreading form and a moderate, regular bearing habit. Trees produce “B” type flowers that shed pollen in the morning. ‘Sharwil’ fruit are oval-shaped and small-seeded, with a green skin, yellow-green flesh and rough skin surface. Typical fruit size is 8 to 10 oz. (Chen et al., 2009). Harvest Maturity ‘Sharwil’ fruit without the stem attached or with blemishes or faults are more likely to be infested by fruit flies (Armstrong, 1991). Mature green fruit, with pedicel firmly attached to trees, should be harvested and packed within 12 h after harvest. Pedicel or fruit stem end length on fruit packed for export should be no longer than 0.5 cm. Fruit should have no surface blemishes or damage. Avocados are classified as mature green fruit if they are hard, meaning the fruit skin and flesh beneath cannot be depressed when applying pressure by hand. Mature green Sharwil avocados are very poor hosts and rarely infested by oriental fruit fly. Fruit without stems and fruit with surface blemishes or damage are more likely to be infested. Harvesting and packing only hard, mature green fruit with stems attached and with no surface blemishes or damage will minimize the risk of exporting infested fruit. Low Pest Prevalence Most systems approaches require low prevalence of fruit flies in the field. After many years of study, only oriental fruit fly has been found to infrequently infest ‘Sharwil’ avocados on the tree. Natural infestations by melon fly and Mediterranean fruit fly have not